As is known to those of skill in the art, frequency shift keying (FSK) is a frequency modulation scheme in which digital information is transmitted through discrete frequency changes of a carrier signal. In Gaussian FSK (GFSK), instead of directly modulating the frequency of the carrier signal with the digital data symbols, instantaneously changing the frequency at the beginning of each symbol, the data pulses are filtered with a pulse shaping Gaussian filter prior to modulating the carrier signal. The Gaussian filter smoothes the transitions between symbols. GFSK is widely used in low data-rate personal communication standards, such as, but not limited to, Bluetooth® and Bluetooth® Low Energy (LE).
The generation of a GFSK signal is illustrated in FIG. 1. A sequence of rectangular pulses 102 representing a data symbol sequence to be transmitted is provided to a Gaussian filter 104 which generates a pulse-shaped version of the data pulses 106. The pulse-shaped signal 106 is then provided to an FSK modulator 108 which modulates the frequency of a carrier signal to generate a GFSK modulated signal 110. The GFSK modulated signal is then provided to a transmitter back-end system where it is up converted to a transmission frequency and coupled to a transmission antenna for radio frequency (RF) transmission.
Since the RF signal received at a GFSK receiver is typically a distorted version of the signal transmitted by the transmitter due to noise etc. the receiver typically has to estimate the transmitted data symbol sequence from the received signal. An example GFSK receiver 200 is shown in FIG. 2. The GFSK receiver 200 comprises an antenna 202, an RF front-end circuit 204 and a baseband circuit 206. The antenna 202 captures an RF GFSK modulated signal and provides the captured signal to the RF front-end circuit 204. The RF front-end circuit 204 down-converts the signal (e.g. via a down-converter 208) to a complex baseband signal and digitises the complex baseband signal (e.g. via an analog to digital converter (ADC) 210) to generate a series of complex (e.g. IQ) samples that represent the received signal. It will be evident to a person of skill in the art that the RF front-end circuit 204 may additionally comprise other components not shown in FIG. 2 to perform other functions such as channel selection, filtering and automatic gain control.
The IQ samples generated by the RF front-end circuit 204 are provided to the baseband circuit 206 which extracts the original data from the IQ samples. The baseband circuit 206 typically comprises a resample circuit 211 which re-samples the IQ samples generated by the RF front-end circuit 204 for processing by the baseband circuit 206; and a detector 212 which is configured to estimate the transmitted data symbol sequence and output the estimated data symbol sequence. Where the original data was encoded by, for example, an error correction code (ECC) the estimated data symbol sequence may be provided to a decoder 214 which is configured to decode the original data stream from the estimated data symbol sequence. It will be evident to a person of skill in the art that the baseband circuit 206 may additionally comprise other components not shown in FIG. 2 to perform other functions such as, but not limited to, carrier frequency offset (CFO) estimation, and timing estimation.
Traditional GFSK detectors are configured to estimate the data symbol sequence by identifying the instantaneous frequency of each IQ sample and translating the identified instantaneous frequency into a data symbol. Where 1/bit per symbol modulation (i.e. 2-GFSK) is used, this means distinguishing between two frequencies—one that represents a 1 and one that represents a 0. The instantaneous frequency is typically identified by a delay and multiply operation. However, this amplifies the noise which limits the sensitivity that can be achieved with such a detector.
The embodiments described below are provided by way of example only and are not limiting of implementations which solve any or all of the disadvantages of known GFSK detectors and/or receivers.